(1) Field of the Invention
The present invention relates to conductive composite materials. More specifically, the present invention relates to high resistivity filler (fiber and/or particle) reinforcements coated with exfoliated graphite particles that are incorporated into the polymer matrix of composite materials to lower resistivity. The exfoliated graphite coating of the fibers and/or particles improves the electrical properties of the resulting composite materials.
(2) Description of Related Art
Nanocomposites composed of polymer matrices with reinforcements of less than 100 nm in size, are being considered for applications such as interior and exterior accessories for automobiles, structural components for portable electronic devices, and films for food packaging (Giannelis, E. P., Appl. Organometallic Chem., Vol. 12, pp. 675 (1998); and Pinnavaia, T. J. et al., Polymer Clay Nanocomposites. John Wiley & Sons, Chichester, England (2000)). While most nanocomposite research has focused on exfoliated clay platelets, the same nanoreinforcement concept can be applied to another layered material, graphite, to produce nanoplatelets and nanocomposites (Pan, Y. X., et al., J. Polym. Sci., Part B: Polym. Phy., Vol. 38, pp. 1626 (2000); and Chen, G. H., et al., J. Appl. Polym. Sci. Vol. 82, pp. 2506 (2001)). Graphite is the stiffest material found in nature (Young's Modulus=1060 MPa), having a modulus several times that of clay, but also with excellent electrical and thermal conductivity. With the appropriate surface treatment, exfoliation and dispersion in a thermoset or thermoplastic polymer matrix results in a composite with excellent mechanical, electrical and thermal properties, opening up many new structural applications as well as non-structural ones where electromagnetic shielding and high thermal conductivity are requirements as well. Furthermore, the economics of producing nanographite platelets indicate a cost of $5 per pound appear to be attainable.
Graphite is a well known material occurring in natural and synthetic form and is well described in the literature. Illustrative of this art is a monograph by Michel A. Boucher, Canadian Minerals Yearbook 24.1-24.9 (1994). A useful form of graphite is expanded graphite which has been known for years. The first patents related to this topic appeared as early as 1910 (U.S. Pat. Nos. 1,137,373 and 1,191,383). Since then, numerous patents related to the methods and resulting expanded graphites have been issued. For example, many patents have been issued related to the expansion process (U.S. Pat. Nos. 4,915,925 and 6,149,972), expanded graphite-polymer composites (U.S. Pat. Nos. 4,530,949, 4,704,231, 4,946,892, 5,582,781, 4,091,083 and 5,846,459), flexible graphite sheet and its fabrication process by compressing expanded graphite (U.S. Pat. Nos. 3,404,061, 4,244,934, 4,888,242, 4,961,988, 5,149,518, 5,294,300, 5,582,811, 5,981,072 and 6,143,218), and flexible graphite sheet for fuel cell elements (U.S. Pat. Nos. 5,885,728 and 6,060,189). Also there are patents relating to grinding/pulverization methods for expanded graphite to produce fine graphite flakes (U.S. Pat. Nos. 6,287,694, 5,330,680 and 5,186,919). All of these patents use a heat treatment, typically in the range of 600° C. to 1200° C., as the expansion method for graphite. The heating by direct application of heat generally requires a significant amount of energy, especially in the case of large-scale production. Radiofrequency (RF) or microwave expansion methods can heat more material in less time at lower cost. U.S. Pat. No. 6,306,264 to Kwon et al. discusses microwave as one of the expansion methods for SO3 intercalated graphite in a solution.
U.S. Pat. Nos. 5,019,446 and 4,987,175 describe graphite flake reinforced polymer composites and the fabrication method. These patents did not specify the methods to produce thin, small graphite flakes. The thickness (less than 100 nm) and aspect ratio (more than 100) of the graphite reinforcement is described.
Graphite, a layered material, is the stiffest material found in nature (Young's Modulus=1060 GPa), having a modulus several times that of clay, but also having excellent electrical and thermal conductivity. As discussed herewith, commonly owned patent applications describe a process using RF or microwave energy to produce exfoliated graphite nanoplatelets (xGnP) which when added to polymers can produce nanocomposites with superior mechanical properties and other desirable properties (e.g. electrical conductivity, thermal conductivity, low permeability, scratch resistance, reduced flammability, etc.) that enhance the use of polymer composites for structural applications such as interior and exterior accessories for automobiles, structural components for portable electrical devices, and non-structural applications where electromagnetic shielding and high thermal conductivity are requirements as well. There is a need to improve this process and composite produced.
Sheet molding compound (SMC) is a composite material currently used in the largest quantities in automotive applications. This composite material is composed of fiberglass-reinforced thermo-set resin made of 3 basic components: the base resin system (polyester, vinylester, epoxy, phenolic or polyimide), the reinforcements (fiberglass, graphite, aramid), and additives that include inert fillers, pigments, UV stabilizers, catalysts, inhibitors, and thickeners. The SCM is not a conductive material and requires extensive surface preparation and the application of a conductive primer prior to painting. Furthermore, its use would be greatly expanded to electromagnetic and radiofrequency shielding applications, if the SMC was electrically conductive.
Fiber reinforced polymer composites have broader application areas due to its higher strength and stiffness. Sheet molding compounding is an industry widely recognized processing method for make composite panel. Generally, two characteristics of molded SMC parts need to be improved, surface finish and conductivity. Conductive SMC can be produced by replacing non conductive fibers with more expensive conductive fibers, addition of large amounts of conductive fillers, of applying a conductive primer before painting. Normally, conductive fillers such as carbon black, carbon fiber, metal whiskers or metal oxide and conductive polymers are dispersed in the SMC resin. U.S. Pat. No. 5,188,783 discloses a method of making a material formed from an ion-conductive polymer and a generally non-ion-conductive polymer. U.S. Pat. No. 4,689,098 discloses a method to prepare a fiber mat reinforced polyphenylene sulfide composite for EMI shielding produced from a thermoformable stampable sheet by distributing nonwoven metal whiskers or fibers of a ductile conductive metal or metal alloy carried-on the mat. U.S. Pat. No. 4,383,942 describes the method of using metal such as aluminum coated glass, metal ribbon and carbon to form conductive thermoset or thermoplastic composite materials. U.S. Pat. Nos. 6,001,919 and 6,814,891 use conductive carbon black to prepare conductive sheet molding compound and finally form a conductive composite part for electrostatic painting. U.S. Pat. Nos. 6,508,906 and 6,901,986 use chopped carbon fiber to prepare conductive sheet molding compound to obtain conductive laminate for further electrostatic painting. In addition, U.S. Pat. No. 7,026,043 invents a method of a combination of chopped fiber and filamentized fiber layer to get resin impregnated filamentized fiber layer as a molded part with improved surface characteristics. It is also mentioned that the resin impregnated filamentized fiber layer may contain a conductive filamentized fiber such that the surface of a sheet molding compound may be conductive and be capable of being electrostatically sprayed.
Conductive compositions can also be formed during compression molding by adding a conductive coating composition onto the surface of composite. U.S. Pat. No. 4,239,808 describes an in-mold coating of sheet molding compound by injecting a coating composition that may include conductive filler, vinyl ester resin, polyepoxide resin with an unsaturated monocarboxylic acid to the SMC cured parts during compression molding to get a smooth surface, fill porosity and other voids and to eliminate or reduce sink marks. U.S. Pat. Nos. 6,872,294 and 6,875,471 use a metallization method to deposit zinc or a zinc alloy onto the polymer composite surface for the purpose of further painting. U.S. Pat. No. 6,001,207 invents a complex method to use a conductive primer including polyester resin containing a fine particulate conductive material such as carbon black to coat and bond to an underlying plastic substrate panel for electrostatic spray painting of the finished contoured panel. U.S. Pat. No. 5,098,771 discloses a method to prepare an electrically conductive composite in a form suitable for applying to the surface of a substrate that includes a polymeric binder into which carbon fibrils are incorporated.
Many patents have been issued related to anode materials for lithium-ion or lithium-polymer batteries (U.S. Pat. Nos. 5,344,726, 5,522,127, 5,591,547, 5,672,446, 5,756,062, and 6,136,474). Among these materials, one of the most widely investigated and used is graphite flakes with appropriate size, typically 2 to 50 μm, with less oxygen-containing functional groups at the edges. Most of the patents described graphite flakes made by carbonization of precursor material, such as petroleum coke or coal-tar pitch, followed by graphitization process.
U.S. Pat. Nos. 4,777,336 to Asmussen et al., 5,008,506 to Asmussen, 5,770,143 to Hawley et al., and 5,884,217 to Hawley et al. describe various microwave or radiofrequency wave systems for heating a material. These applications and patents provide a background technology for the novel graphite exfoliating process preferred for the present invention.